The invention relates to image forming apparatuses performing an electrophotographic image forming process by forming an electrostatic latent image on a circumferential surface of a photoreceptor. The invention relates in particular to a method of preventing a cleaning blade, which is provided in a cleaning unit of the apparatuses for removing residual developer from the circumferential surface of the photoreceptor, from becoming warped.
In electrophotographic image forming apparatuses, an electrostatic latent image is formed on a circumferential surface of a photoreceptor according to image data. The electrostatic latent image is developed with developer into a developer image. Then, the developer image is transferred onto a recording medium such as a sheet of paper. In such image forming apparatuses, a charging process and an exposing process are performed in the mentioned order. In the charging process, a charging unit charges the circumferential surface of the photoreceptor so that the surface has a uniform charge distribution thereon. In the exposing process, an exposing unit irradiates the circumferential surface with image light modulated according to image data. In the exposing process, the circumferential surface of the photoreceptor is partially irradiated and discharged, with image light. Thus, an electrostatic latent image is formed on the circumferential surface, with a discharged portion as a black-image portion and a nondischarged portion as a white-image portion, respectively.
Rotation of the photoreceptor drum brings the electrostatic latent image to a developing area. To the developing area, developer is supplied from a developer sleeve provided in a developing unit. The developer consists of, or includes, toner. The toner is electrostatically attracted to the black-image portion of the electrostatic latent image because of a difference in potential between a developing bias voltage applied to the developer sleeve and a potential that the latent image has. The latent image is thus developed into a toner image.
Upon receipt of a request for image formation, sheets are fed, one at a time, from a sheet feeding section to an image forming section that includes a photoreceptor. Registration rollers are provided immediately upstream of the image forming section. A sheet is transported to a transfer area by the registration rollers so that a leading end of the sheet meets in a timely manner a leading end of a toner image formed on the circumferential surface of the photoreceptor. In the transfer area, the circumferential surface and a transferring unit faces each other. In the transfer area, the transferring unit applies a voltage opposite in polarity to a charge of the toner, so that the toner image is transferred onto the sheet.
In the transfer process as described above, toner that forms the toner image is not all transferred from the circumferential surface of the photoreceptor to the sheet. 5 to 15 percent of the toner remains on the circumferential surface. 100% transfer efficiency is not obtained because some of the toner is oppositely charged and because some of the toner, once transferred to the sheet, is transferred back to the photoreceptor when the sheet is separated from the photoreceptor.
Since the photoreceptor is rotated, image forming processes are performed numerous times on the circumferential surface of the photoreceptor. If an image forming process is performed with residual toner remaining on the circumferential surface, the circumferential surface is prevented from being uniformly charged in the charging process. Thus, residual toner contributes to poor image quality. In view of the foregoing, a cleaning unit is provided downstream of the transfer area in order to remove residual toner from the circumferential surface of the photoreceptor.
The cleaning unit usually includes a blade and a screw. The blade, which is in contact with the circumferential surface of the photoreceptor, is provided for scraping off residual toner into a toner collecting portion. The screw is provided for blowing away the residual toner as scraped off. The blade is usually made of hard rubber. The blade is pressed against the circumferential surface of the photoreceptor at a predetermined pressure. Stick-slip motions of the blade caused by rotation of the photoreceptor serve to flick the residual toner off the circumferential surface into the toner collecting portion. The blade is in contact at an edge thereof with the approximately full width of the circumferential surface along a fast scanning direction, i.e., a direction perpendicular to a direction in which the circumferential surface moves.
If a sheet being transported has a smaller length than the width of the photoreceptor along the fast scanning direction, the contact edge of the blade is more likely to be dragged in the direction in which the circumferential surface moves, so that the blade is more likely to become warped.
Residual toner remains on the image area of the circumferential surface, where a sheet faces the photoreceptor, after the transfer process is completed. A toner particle has mobility and also has a larger diameter than a water molecule. The blade has toner adhering to a contact edge thereof, thereby being prevented from becoming warped despite a high water content in the air.
In contrast, there is no residual toner on a non-image area of the circumferential surface where a sheet does not face the photoreceptor and thus a toner image is not formed. Accordingly, the high water content in the air renders the contact edge less likely to slip on the circumferential surface, particularly in a situation in which toner has not been present in the non-image area in consecutive image forming processes. In such a situation, vibration of the blade caused by rotation of the photoreceptor causes a contact-edge side of the blade to be dragged in the direction in which the circumferential surface of the photoreceptor is rotated, so that the blade becomes warped.
To deal with the foregoing problem, JP H05-150696 A discloses an image forming apparatus in which a blade provided in a cleaning unit is pressed against a circumferential surface of a photoreceptor at an appropriate pressure (torque) such as to prevent the blade from becoming warped. In an image forming apparatus disclosed by JP H01-229281 A, a black solid image is formed across the full width of a circumferential surface of a photoreceptor. The blade has contact with the solid image, so that toner adheres to the full width of a contact edge of the blade. The blade is thus prevented from becoming warped.
In the apparatus as disclosed by JP H05-150696 A, the blade is prevented from becoming warped, by setting appropriate torque for a material thereof according to prestored information on torque setting. It is impossible that the prestored information covers all of diverse materials, and thus a blade of a novel material may not be prevented from becoming warped. If the blade changes in properties over time, furthermore, appropriate torque to be applied to the blade may also change. Accordingly, it is difficult to ensure that the blade is prevented from becoming warped for a long time period.
The apparatus disclosed by JP H01-229281 A has the following problems. Forming a black solid image across the full width of the circumferential surface involves consumption of a large amount of toner, thereby causing an increase in running cost. Also, toner as collected has to be frequently removed from the apparatus. Such frequent toner removal prevents the apparatus from operating efficiently.
It is a feature of the invention to provide an image forming apparatus configured to prevent a blade from becoming warped, without causing an increase in running cost or in frequency of maintenance, by additionally forming on a circumferential surface of a photoreceptor a toner image of low density that is not transferred to a record medium so that toner constantly adheres to an edge of the blade.